Bacteria have different dye affinity depending upon different compositions of the cell wall when tested with Gram staining and are divided into two major groups of Gram-negative and Gram-positive bacteria based on differences in their dye affinity. In case of Gram-negative bacteria, the cell wall consists of an outer membrane and a peptidoglycan layer on the inside of the outer membrane. The outer membrane consists of phospholipids, lipopolysaccharides (LPS), lipoproteins and membrane proteins. A unit membrane structure of the outer membrane consists of phospholipids and lipopolysaccharides. A lipopolysaccharide is found in an outer layer of the membrane and a phospholipid in an inner layer thereof. A lipopolysaccharide consists of a high molecular weight lipid, called Lipid A, and a polysaccharide bound thereto. Lipid A makes up an outer layer of the outer membrane whereas a polysaccharide extends outwards from the outer membrane. The kind of saccharides is different from each other between the outer portion and the portion at the vicinity of Lipid A. The outer portion is called an O-side chain polysaccharide (O-antigen) and the inner portion is called a core polysaccharide. The saccharide consisting of O-antigen is chiefly a hexose and a pentose and a basic structure consisting of 3 to 5 kinds of these saccharides appear repeatedly. In the core polysaccharide are present, in addition to these saccharides, saccharides unique to the respective bacteria such as an octose, e.g. ketodeoxyoctonate, and a heptose. Lipid A is a lipid unique to the respective bacteria which comprises a saccharide, i.e. two molecules of glucosamine which are β-1,6-bound to each other, and phosphoric acid and a fatty acid bound to said saccharide. Lipid A is bound to a core polysaccharide at 6′-position of the saccharide.
There are many kinds of Gram-negative bacteria. Salmonella and E. coli, belonging to Enterobacteriaceae, and Haemophilus, belonging to Pasteurella, are also included in Gram-negative bacteria.
Salmonella, secondary large bacillus with peritrichous flagella, is divided into groups by the type of O-antigen and is further subdivided by the types of H-antigen, resulting in more than 2,000 kinds of serotype. A host range of Salmonella is quite wide and a variety of mammals including human and birds are known to be infected with or to hold Salmonella. When chicken are infected, Salmonella may cause septic diseases in young chicks. In case of adult chickens, however, carrier chickens are asymptomatic to escape culling and as a result chicken meat and eggs derived from chickens contaminated with Salmonella are distributed to induce food poisoning in human through food products manufactured thereby.
Food poisoning by Salmonella develops after a latent period of 12 to 48 hours after intake of contaminated food. A latent period may vary depending on an intake amount of bacteria, the condition and age of patients. Symptoms are mainly acute gastroenteritis and cardinal symptoms are diarrhea, abdominal pain, vomiting and fever. Thus, a salmonella vaccine for chicken is not preventing chickens from the disease onset but is an important vaccine used for public health.
Among conventional vaccines against Salmonella is a whole-cells vaccine which comprises cells of inactivated Salmonella. A whole-cells vaccine, however, may cause side effects since it contains portions that are not of antigenicity. In case of chickens that are bred in flocks, in view of labor saving of vaccination, there is a strong demand for a multivalent vaccine that may prevent many diseases with a single injection. Also, a multivalent vaccine may contribute to reduction of stress in chickens since it reduces the number of injections. However, although a multivalent vaccine has such convenience, it is liable to cause vaccination reaction at the injection site, in particular, when it contains bacteria such as Salmonella. 
Under the circumstances, research of a component vaccine against Salmonella has been conducted, among which application of a lipopolysaccharide (O-antigen) is studied. For instance, there is a report that a conjugate consisting of O-antigen derived from Salmonella Typhimurium bound to a carrier protein is administered to mice to confirm its efficacy (Non-patent reference 1). There is also a report that a lipopolysaccharide derived from Salmonella Typhi is administered to mice to confirm its efficacy (Patent reference 1). Other than Salmonella, there is a report that a lipopolysaccharide derived from Burkholderia thailandensis or Burkholderia pseudomallei is administered to mice to confirm its efficacy (Patent reference 2, Non-patent reference 2).
However, contamination of a lipopolysaccharide is avoided with meticulous care in a medicament used for living body since a lipopolysaccharide may clinically cause a variety of highly lethal diseases such as septic shock, disseminated intravascular coagulation (DIC) and multiple organ failure (MOF) and may become a causal factor of fever even in a trace amount (Non-patent reference 3). In practice, it is reported that more than 90% of mice die within 72 hours when administered with a lipopolysaccharide (Patent reference 3). Under the circumstances, an idea to use a lipopolysaccharide per se as a vaccine antigen would not arise generally.